This invention relates to blow molding and more particularly to method improvements in starting up a multi-station, rotary blow molding machine.
Vertically disposed, rotary blow molding machines having a plurality of circularly arranged molds equi-spaced from each other and adapted in operation to continuously revolve through a circular path are well known in industry and desirable versus alternative types largely because start-stop intermittent motion of the usually substantial mass of the machine is avoided. The molds successively close on portions of a deformable thermoplastic parison continuously extruding downwardly in an area generally tangential to the circular path of mold movement, and the formed article is ejected from the molds at another area of such path after being blown and cooled therein while the mold was closed and was traversing a significant portion, e.g. on the order of 270.degree., of such circular path. In this last respect, it is desirable to close each mold as soon and as close as possible to the head where the parison is formed in order to immediately blow it against the cooled walls of the mold cavity and then hold it in contact therewith for as long as possible before discharge so that the plastic will be well set and not subject to any substantial distortion during or after ejection. Available space in the area immediately around the extruder orifice where the sections are continuously successively closing on each other is therefore generally non-existent.
If extrusion rate is out of phase with the rate at which the molds are charged or loaded, problems result in that if extrusion is too slow with respect to machine speed, the portions of the continuous parison between successive captured sections will stretch and frequently erratically break during loading and this requires rethreading, i.e. concentrically orienting the free swaying end of the next issuing portion with respect to the oncoming mold, usually manually by hand, which can be a serious safety problem in the aforementioned cramped area adjacent the extruder outlet. On the other hand, if extrusion rate substantially exceeds rotary mold velocity, the extrudate can undesirably build up on the top face of each closing mold before the next one to be loaded moves into place. If the extrusion rate is brought up to operating level before the molding machine is started, it has been customary to manually break off lengths of the extrudate as it exits the extrusion nozzle until the linear rate is at the desired level, whereupon the machine is started and the free end presented to the first closing mold in the sense of being guided into place with a pair of hands, which is rather dangerous from a safety standpoint. In this last respect, if the length is inadequate for clamping on both ends by the mold sections, the captured portion cannot be blown since one end is not sealed by the sections and, in the past, it has been difficult to remove such non-blown (or under-blown) parison portions from the mold, frequently requiring that such removal also be done manually. The aforementioned problems are aggravated when more than one parison is being formed simultaneously.